The Syrian hamster has a peculiar sex limited serum protein, expressed as a major protein in females (therefore called Female Protein) and testosterone suppressed in males. Female Protein (FP) is a homolog of two human pentraxins, C-reactive protein (CRP) and amyloid P component (AP) as shown by similar structure (pentameric) and amino acid sequence. Furthermore, FP shares function-properties with both human pentraxins such as Ca++ dependent phosphorylcholine binding, complement fixation, acute phase responsiveness (characteristics of CRP) and also is a constituent of amyloid (characteristic of AP). Indeed, high serum levels of FP occurring naturally (as in female) or experimentally (as in hormonally treated male) are directly associated with deposition of amyloid. Whether these high serum FP levels are directly responsible for amyloid deposition is unknown at present. However, by following the metabolism of injected I125I-FP, an extraordinary sequestration of serum I125-FP can be demonstrated within amyloid deposits. This diagnostic alteration of a serum protein (FP) metabolism in the amyloidotic hamster has not been observed in other amyloid models. The mechanism is still unknown, although it indicates a dynamic exchange of FP between serum and amyloid compartments. Present experiments are designed to limit and reverse amyloid deposition by control of serum FP levels and alteration of FP binding capacity. Although FP is a major serum component (of females) and represents an ancient protein which has changed little during evolution, its reason for existence is unknown.